1. Field of the Invention
The present invention relates to a steel used for hot forged formed articles which show little deformation during fracture, and which have excellent fatigue strength. The present invention relates to a steel for machine structural use which can be used without quenching and tempering after forming by hot forging.
2. Description of the Prior Art
In general, machine structural parts are completed by hot forging a steel material, and further conducting quenching and tempering, and machining. However, in recent years, a steel which is not quenched and tempered subsequent to forming by hot forging, namely, a non-heat treated steel for hot forging (hereinafter referred to as a non-heat treated steel) which realizes excellent mechanical properties in an as-hot-forged state has been widely used. Use of the non-heat treated steel results in reduction of the cost by omission of the heat treatment step, and overcoming the problem of quenching strain by omission of quenching.
On the other hand, in order to further obtain the cost-reduction merit, new working methods have been devised. A typical example thereof is a method for working a connecting rod which transmits the explosion force of an engine to a crank shaft. A connecting rod has heretofore been prepared by integrally or separately forming its cap portion and rod portion by hot forging, and finishing by machining. However, there has recently been adopted a method wherein the rod and the cap portion are fracture split by impact stretching, and the fracture surfaces are butted against each other, and joined together, because joining the cap and the rod portion requires working with high precision or because of the like matter. The non-heat treated steel for fracture split generally used is a steel containing about 0.7% of C. Deformation during fracture is suppressed and rejoining is made easy by using a high carbon steel.
On the other hand, the steel having a high carbon composition has a disadvantage of a low yield ratio and a low fatigue strength ratio, and it has a problem that its machinability is deteriorated when the tensile strength is increased to obtain a high yield strength and a high fatigue strength.
Furthermore, a number of non-heat treated steels having a relatively low carbon content and excellent fracture splitability are disclosed in Japanese Unexamined Patent Publications (Kokai) No. 9-268345, No. 9-31046, and the like. For example, the non-heat treated steel for hot forging disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-268345 contains less carbon (0.35 to 0.60%) than the currently used steel; the patent publication describes a method for producing a connecting rod wherein a notch groove having a stress concentration factor of at least 2 is provided to a connecting rod to be treated, and it is fracture split by impact load. Japanese Unexamined Patent Publication (Kokai) No. 9-310146 discloses a steel which similarly contains less carbon (0.30 to 0.60%) than the currently used steel, and in which a combination of elements for improving the proof stress and workability is restricted.
However, although these steels show ordinary mechanical properties and fracture splitability, they have a problem that they cannot respond to severe industrial requirements, particularly to the requirements of improving the fatigue strength.